The invention relates to vinyl acetate-ethylene copolymer emulsions and, more particularly, relates to high solids copolymer emulsions. High solids vinyl acetate/ethylene emulsions are characterized as having a solids level of at least 65% to about 75% by weight and having a viscosity of less than 5000 centipoises. Stabilization of high solids, e.g., vinyl acetate/ethylene (VAE) based copolymers prepared by aqueous emulsion polymerization typically has been achieved by the combined use of poly(vinyl alcohol) and nonionic surfactants.
Representative patents showing the production of vinyl acetate/ethylene emulsions and high solids vinyl acetate/ethylene emulsions are as follows:
U.S. Pat. No. 3,218,281 discloses a process for producing vinyl acetate emulsions in the presence of a water soluble emulsifying agent consisting of copolymers of vinyl acetate and polyoxyalkylene compounds, such as copolymers of vinyl acetate and polyoxyethylene glycol.
U.S. Pat. No. 4,133,791 discloses the production of ethylene containing copolymers in the presence of poly(vinyl alcohol) having a molecular weight of from 6000 to 100,000 to a solids content of from 30 to 60% by weight and then evenly distributing polyethylene glycol or a mixture of polyethylene glycol; and polyethylene/propylene glycol in the copolymer dispersion. The resultant viscosity of the dispersion is above 20 Pas.
U.S. Pat. No. 4,921,898 discloses a process for producing high solids VAE compositions by using a stabilizing amount of a poly(vinyl alcohol)/surfactant system consisting essentially of a poly(vinyl alcohol) having a 100-600 degree of polymerization and a nonionic surfactant consisting of a 30-40 ethylene oxide unit substituted alkyl phenol or a 30-40 ethylene oxide unit substituted propylene glycol.
U.S. Pat. No. 4,963,611 discloses the production of vinyl acetate emulsions for use as adhesives and paints employing a water soluble poly(vinyl alcohol) as a stabilizing agent. Example 4 shows the production of a low solids vinyl acetate homopolymer in the presence of poly(vinyl alcohol) and a polyethylene glycol having a molecular weight of 200.
U.S. Pat. No. 5,070,134 disclose a vinyl acetate/ethylene high solids emulsion utilizing a stabilizing system comprised of from 1 to 1.9 parts of poly(vinyl alcohol) having a degree of polymerization from 300 to 1000 and from 1 to 3 parts by weight of a polyoxyethylene nonionic surface active agent having an HLB of 15.5 to 17.5 per 100 parts by weight vinyl acetate.
U.S. Pat. No. 5,110,856 discloses high solids vinyl acetate/ethylene emulsions formed by polymerizing vinyl acetate and ethylene in the presence of a stabilizing system of poly(vinyl alcohol) having a degree of polymerization of 300 to 1000 and from 1 to 3% of a polyoxyethylene surface active agent having an HLB of 16.5 to 17.5.
U.S. Pat. No. 5,629,370 discloses a vinyl acetate/ethylene copolymer emulsion having a solids content of at least 65% by weight. The emulsion is prepared by polymerizing vinyl acetate and ethylene in the presence of a partially hydrolyzed poly(vinyl alcohol) and a mixture of nonionic ethoxylated alkyl phenol surfactants having an HLB of from 16 to 16.5.
EP 0 089 068 discloses a process for producing high solids  greater than 70% a viscosity of 5000 mPaxc2x7s or less utilizing a stabilizing system comprised of poly(vinyl alcohol) having a degree of hydrolysis of 88% and a polyoxyethylene oxide surfactant such as that sold under the trademark Pluronic.
The present invention provides for the production of stable aqueous dispersions of vinyl acetate-ethylene copolymers having a solids content typically from 55 to 75% by weight and higher, preferably emulsions having a high solids content, i.e., greater than 65 wt %, preferably  greater than 70% which are useful as adhesives. They demonstrate surprisingly good speed of set and creep resistance at such high solids level. The vinyl acetate-ethylene copolymer is comprised of polymerized units of vinyl acetate and ethylene in approximately 50-95 wt % and 5-50 wt %, respectively. Optionally, other monomers may be copolymerized therewith.
The copolymers are prepared by the emulsion copolymerization of vinyl acetate and ethylene monomers in the presence of a stabilizing system consisting essentially of (1) poly(vinyl alcohol) which is 75-99+mole % hydrolyzed and has an average degree of polymerization ranging from 100-2200 and (2) a polyethylene glycol having a number average molecular weight of from 200 to 20,000.
The resulting vinyl acetate/ethylene copolymer emulsions will comprise about 55 to about 75 wt % solids, preferably from 65 to 75% solids with a viscosity of less than about 5000, typically less than 3500 cps, preferably less than about 2500 cps at 65% solids or greater and at 60 rpm (Brookfield #4 spindle at 25xc2x0 C.).
Several advantages can be achieved by the invention:
increased polymer solids at viscosities comparable to lower solids emulsions;
excellent resistance to sedimentation and excellent stability of the emulsion;
accelerated bond formation and adhesive setting speed; and,
improved creep resistance versus many conventional high solids emulsions.
The copolymers according to the invention typically comprise from 50-95 wt % vinyl acetate and 5-50 wt % ethylene to provide a Tg ranging from about xe2x88x9235 to 20xc2x0 C. Preferably, the copolymer forming adhesive applications contains from 65 to 80 wt % vinyl acetate and 20 to 35 wt % ethylene on a monomer basis. Pressure sensitive adhesive applications may contain ethylene concentrations from about 35 to 45%.
The vinyl acetate-ethylene copolymers may optionally include one or more additional ethylenically unsaturated copolymerizable monomers. Exemplary of such comonomers, which may be present in amounts from 0 to 10 wt %, but preferably less than 5%, e.g., 0.5 to 5% are C3-C10 alkenoic acids, such as acrylic acid, methacrylic acid, crotonic acid and isocrotonic acid and their esters with C1-C18 alkanols, such as methanol, ethanol, propanol, butanol, and 2-ethylhexanol; vinyl halides such as vinyl chlorides; alpha, beta-unsaturated C4-C10 alkenedioic acids such as maleic acid, fumaric acid and itaconic acid and their monoesters and diesters with the C1-C18 alkanols, such as dibutyl maleate and dioctyl maleate; and nitrogen containing monoolefinically unsaturated monomers, particularly nitriles, amides, N-methylol amides, lower alkanoic acid esters of N-methylol amides, lower alkyl ethers of N-methylol amides and allylcarbamates, such as acrylonitrile, acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N-methylol allylcarbamate, and lower alkyl ethers or lower alkanoic acid esters of N-methylol acrylamide, N-methylol methacrylamide and N-methylol allylcarbamate. If any additional ethylenically unsaturated comonomer is used, about 1-5 wt % is preferred.
The stabilizing system for the copolymerization reaction consists essentially of poly(vinyl alcohol) and polyethylene glycol wherein the poly(vinyl alcohol) is present in an amount of from 1-4 wt %, preferably 2 to 3 wt % by weight of copolymer and from 1-4 wt %, preferably 2-3 wt % of the polyethylene glycol, said polyethylene glycol having a number average molecular weight of from 200 to 20,000 by weight of the copolymer. (Typically, the poly(vinyl alcohol) and polyethylene glycol are based upon the total weight of the monomers charged to the polymerization.) The poly(vinyl alcohol) which is used in the stabilizing system can be 75-99+mole % hydrolyzed, preferably 85-90 mole % and especially 87-89 mole % hydrolyzed and has a degree of polymerization ranging from 100 to 2200, preferably, 140 to 1000, and most preferably 200 to 700. Another means for assessing the degree of polymerization of the poly(vinyl alcohol) is its viscosity as a 4 wt % aqueous solution at 20xc2x0 C. Suitable poly(vinyl alcohol)s will have a viscosity ranging from 2 to about 50, preferably 2.5 to 7 for higher solids emulsions.
As stated the stabilizer system according to the invention also contains a polyethylene glycol at a level from about 1-4 wt %, preferably 2-3 wt %, based on copolymer. Polyethylene glycols are widely known and include those materials sold under the trademark Carbowax. They should have a number average molecular weight of from 200 to 20,000. When producing emulsions having conventional solids level, e.g., 55 to 60% by weight, a wide range of polyethylene glycols may be used as stated with preferred polyethylene glycols having a molecular weight from 600 to 8000. High solids emulsions of greater than 65% solids having excellent stability and employ a stabilizing system of poly(vinyl alcohol) and polyethylene glycol with preferred polyethylene glycols having a molecular weight of from about 500 to 2000. The polyethylene glycols permit the formation of emulsions having substantially lower viscosity.
The stabilizing agent consisting essentially of the poly(vinyl alcohol) and polyethylene glycol should be used in an amount of from about 3 to 8%, preferably 4 to 6% by weight of the total monomer polymerized. If a lesser amount is employed, the system may coagulate because of insufficient stabilization.
The weight ratio of poly(vinyl alcohol) to polyethylene glycol should range from about 40 to 60 parts poly(vinyl alcohol) to 60 to 40 parts polyethylene glycol on a 100 weight basis of total stabilizer system. The balance should consist of the polyethylene glycol. Optionally, small amounts, e.g., 5 to 20% by weight of the total stabilizing system may be made up of other conventional, and compatible, surfactants. Generally, though it is not necessary to add further surfactants to produce a stable emulsion.
Various free-radical forming sources can be used in carrying out the polymerization of the monomers, such as peroxide compounds. Combination-type systems employing both reducing agents and oxidizing agents can also be used, i.e. a redox system. Suitable reducing agents, or activators, include bisulfites, sulfoxylates, or other compounds having reducing properties such as ascorbic acid, erythorbic acid and other reducing sugars. The oxidizing agents include hydrogen peroxide, organic peroxide such as t-butyl hydroperoxide and the like, persulfates, such as ammonium or potassium persulfate, and the like. Specific redox systems which can be used include hydrogen peroxide and zinc formaldehyde sulfoxylate; hydrogen peroxide and erythorbic acid; hydrogen peroxide, ammonium persulfate or potassium persulfate with sodium metabisulfite, sodium bisulfite, ferrous sulfate, zinc formaldehyde sulfoxylate or sodium formaldehyde sulfoxylate.
The oxidizing agent is generally employed in an amount of 0.01-1%, preferably 0.05-0.5%, based on the weight of the vinyl acetate introduced into the polymerization system. The reducing agent is ordinarily added in an aqueous solution in the necessary equivalent amount.
In general, suitable vinyl acetate-ethylene copolymer emulsions can be prepared by the copolymerization of the monomers in the presence of the poly(vinyl alcohol)-polyethylene glycol stabilizing system in an aqueous medium under pressures up to about 100 atm and in the presence of a redox system which is added incrementally, the aqueous system being maintained by a suitable buffering agent at a pH of about 2-6. The process first involves homogenization in which the vinyl acetate suspended in water is thoroughly agitated in the presence of ethylene under the working pressure to effect solution of the ethylene in the vinyl acetate while the reaction medium is gradually heated to polymerization temperature. The homogenization period is followed by a polymerization period during which the redox system is added incrementally.
The reaction temperature can be controlled by the rate of redox addition and by the rate of heat dissipation. Generally, it is advantageous to maintain a mean temperature of about 55 to 90xc2x0 C. during the polymerization of the monomers. While temperatures as low as zero degrees can be used, economically the lower temperature limit is about 50xc2x0 C.
The reaction time will depend upon the variables such as the temperature, the free radical forming source and the desired extent of polymerization. It is generally desirable to continue with the reaction until less than 0.5% of the vinyl acetate remains unreacted. While the reaction time of the polymerization process will vary as mentioned above, the use of the stabilizing system according to the invention not only provides a high solids vinyl acetate-ethylene copolymer emulsion but also provides for emulsion polymerization in a surprisingly shorter reaction time, i.e. the polymerization reactor cycle time is significantly decreased by virtue of higher polymerization temperatures.
In carrying out the polymerization, an amount of the vinyl acetate is initially charged to the polymerization vessel and saturated with ethylene. Most advantageously, at least about 50% of the total vinyl acetate to be polymerized is initially charged and the remainder of the vinyl acetate is added incrementally during the course of the polymerization. The charging of all the vinyl acetate initially is also contemplated with no additional incremental supply. When reference is made to incremental addition, substantially uniform additions, both with respect to quantity and time, are contemplated. Such additions can be continuous or discontinuous and are also referred to as xe2x80x9cdelayxe2x80x9d additions.
The quantity of ethylene entering into the copolymer is influenced by pressure, the agitation (mixing) and viscosity of the polymerization medium. Thus, to increase the ethylene content of the copolymer, high pressures, greater agitation and a low viscosity are employed.
The process of forming the vinyl acetate-ethylene copolymer emulsion generally comprises the preparation of an aqueous solution containing the stabilizing system and, optionally, the pH buffering system. This aqueous solution and the initial or total charge of the vinyl acetate are added to the polymerization vessel and ethylene pressure is applied to the desired value. The pressurized ethylene source can be shut off from the reactor so that the ethylene pressure decays as it is polymerized or it can be kept open to maintain the ethylene pressure throughout the reaction, i.e. make-up ethylene. As previously mentioned, the mixture was thoroughly agitated to dissolve ethylene in the vinyl acetate and in the water phase. Conveniently, the charge is brought to polymerization temperature during this agitation period. The polymerization is then initiated by introducing initial amounts of the oxidant, the reductant having been added with the initial charge. After polymerization has started, the oxidant and reductant are incrementally added as required to continue polymerization. Any third copolymerizable monomer and the remaining vinyl acetate, if any, may be added as separate delays.
As mentioned, the reaction is generally continued until the residual vinyl acetate content is below about 0.5%. The completed reaction product is then allowed to cool to about room temperature while sealed from the atmosphere. The pH is then suitably adjusted to a value in the range of 4.5 to 7, preferably 4.5 to 5, to insure maximum stability.
Vinyl acetate-ethylene copolymer emulsions can be directly produced having a solids content of about 55 to 75%, and preferably from 65-75% sand having a viscosity of less than about 5000 cps, generally less than 3500 cps and preferably less than about 2500 cps and most desirably less than about 1500 cps, at 65% solids and at 60 rpm and 25xc2x0 C.